Actives glossary
L-ascorbic acid
The best documented vitamin C and the least stable, which is why the packaging matters as much as the percentage.
- INCI name
- Ascorbic Acid
- Also called
- vitamin C, LAA
- Family
- Vitamin
- Range seen on labels
- 10% to 20%, at a pH below 3.5
- Sits well with
- zinc-oxide
- Needs care with
- niacinamide, retinol
L-ascorbic acid is the form of vitamin C with the most direct evidence behind it, and the form that degrades fastest. It needs a low pH to penetrate, and that same low pH is where it oxidises most readily once air reaches it.
The colour tell, again
An L-ascorbic acid serum that has turned deep yellow or brown has oxidised. Unlike the yellow of retinaldehyde, which is the ingredient’s own colour, this is a sign the product has moved on. It is one of the few honest at-home tests in skincare.
Derivatives
Magnesium ascorbyl phosphate, sodium ascorbyl phosphate, ethyl ascorbic acid and ascorbyl glucoside are more stable and less studied. They are not interchangeable with L-ascorbic acid at the same percentage, and a product that lists a derivative but talks about the L-ascorbic acid literature is quietly changing the subject.